A refining furnace or oven structure is often constructed of refractory bricks lining the inside of a metal frame. The furnace structure includes a bottom portion comprising refractory brick and other refractory lining materials so arranged as to form a leak-tight reservoir wherein a molten metal resides. The structure also includes a roof portion and a sidewall portion. The bottom, roof and sidewall portions form the receiving volume for a refinable metal charge and isolate and insulate an interior heat source. A typical heat source is a natural gas flame.
Refractory brick is composed of various refractory oxides including magnesium, chromium, aluminum and silicon oxides which are formed in the shape of rectangular parallelepipeds. Bricks are supported by various means depending on the structure constructed. The bottom portion of the furnace structure is comprised of bricks positioned with one end exposed to the furnace interior and the other end engaging and being supported by the metal furnace frame. Bricks comprising the sidewall portion may be supported by the metal furnace frame with sufficient layers of bricks stacked one upon another to provide a desired wall height. Alternatively, wall portions may be built up from bricks with integral metal hangers attached to the metal frame over the entire height of the sidewall. Bricks comprising the roof portion may be supported with metal plates as is disclosed in U.S. Pat. No. 4,529,178, wherein, bricks supported by metal plates are hung from metal support members.
Portions of brick exposed to the heat source and molten metal are subject to corrosion, wear and mechanical damage. Extreme temperatures, corrosive slag and chemically reactive atmospheres containing, for example, sulphur, carbon monoxide and carbon dioxide and various acid fumes all act to burn away the brick material. Mechanical damage compounds with thermochemical wear and results in portions of the furnace wearing more quickly than other portions. For example, the sidewall portion of the furnace under the influence of high temperature, corrosive atmosphere and mechanical damage caused, for example, by the introduction of a furnace charge, wears more quickly than the roof portion. Wear may proceed to a point wherein the worn portion may rupture. When rupture occurs, flames originating from the interior heat source may escape, or if the rupture is below the surface of the molten charge, molten metal will leak from the furnace.
Prior art repair techniques include those where maintenance personnel enter the furnace and rebuild damaged or worn portions. The disadvantages of this method are that the entire furnace must be emptied and cooled to permit safe entry by the maintenance personnel and that the time required for the repair is lengthy. Because of the furnace's large thermal mass and a typical operating temperature in excess of 2000.degree. F, the time required to cool the furnace can be in excess of two weeks. Other methods allow direct access to the interior of the furnace by personnel without a complete furnace cool-down. Such a method which is disclosed in U.S. Pat. No. 4,452,749 necessitates the cooling of the furnace or oven to a brick surface temperature of about 500.degree. F. Maintenance personnel wearing protective clothing with integral cooling systems enter the oven and repair portions of the furnace brick structure. This method is potentially hazardous and requires substantial cooling and the subsequent reheating of the furnace.
Other methods of repair include those that do not require furnace entry directly by repair personnel. An external access method includes a gunning technique wherein a refractory repair material is sprayed into the damaged or worn region as disclosed in U.S. Pat. Nos. 4,779,798 and 4,465,648. Although there appears to be no requirement of furnace cool down to effect this method, use of the gunning technique results in non-uniformity of the repaired portions and shorter useful life than the original refractory material.
Another external access method is disclosed in U.S. Pat. No. 4,017,960 where a damaged portion is removed from the furnace by cutting through a steel support structure and through the damaged refractory portion. The cut is made approximately around the perimeter of the damaged or worn portion. The damaged or worn portion described is removed and replaced by a section of similar refractory material which has been cut in a shape complementary to the shape of the opening remaining in the furnace shell and refractory lining. This method disadvantageously requires a custom shaping of the section which is inserted into the opening of the furnace sidewall.
What is needed and what does not appear to be provided in the prior art is a furnace which is constructed so as to facilitate repair. Generally, the sought-after furnace should be one for which repairs may be made from outside the furnace without the need to cool down completely the furnace during such repair.